The invention relates to a method for controlling access to a motor vehicle. In particular, the invention relates to a method for checking the legitimacy of access, in which a keyless access system (keyless entry) is used.
Keyless access systems allow legitimate users of a motor vehicle to initiate a legitimacy check in the motor vehicle without mechanical action on the motor vehicle and merely by the approach of wireless communication means to the motor vehicle. The user carries an ID transponder (e.g., a radio vehicle key) having a control unit integrated therein. The ID transponder-side control unit is coupled to a transmission-reception means. On the vehicle side, a vehicle-side control device having an associated transmission-reception means is present.
An ID transponder and a motor vehicle exchange information between the respective control units.
Signals are often transmitted both in the low frequency range (a few to several hundred KHz) and in the high frequency range (several hundred MHz to several GHz) for various functions. In this case, the signal transmission in the low frequency range is of shorter range than in the high frequency range. Thus, it is to be ensured, among other things, that only users having a matching ID transponder in the immediate vicinity of the motor vehicle get access to the motor vehicle.
It is known that access to a motor vehicle can be compromised by so-called relay-station attacks.
For example, US 2006/0255909 A1 discloses a system for preventing such relay-station attacks.
In such an attack, a great distance between an ID transponder (e.g., in the form of a motor vehicle key) and the motor vehicle is bridged, even though access control at this distance should not actually be successfully transacted.
For example, two people approach, each equipped with radio-extension stations, to the active components of the access system. In this case, one person approaches the motor vehicle, whereas the other person approaches the ID transponder. Both persons carry electronic radio-extension stations that are in high frequency communication with each other.
In addition, these extension stations are able to receive both low frequency- and high frequency signals from the motor vehicle or from the ID transponder. These signals are to be transmitted in a high frequency message to the other station and to be output from there again as low frequency or high frequency signals.
As described in the aforementioned document, this leads to, for example, allowing access to a motor vehicle, even though a user with a legitimate ID transponder is far away from the associated motor vehicle.
The entire low frequency (LF) information flow, and also the subsequent high frequency communication between the ID transponder and a motor vehicle is extended via the two extension stations as relay stations.
Document WO 2015/084852 discloses a system and a method, which is to prevent the extension by such relay attacks. This system works by analyzing the field vectors of the signal fields and the angles between the field vectors, in which the associated fields are generated by multiple antennas on a motor vehicle.
The antennas on the motor vehicle are time-delayed or also activated at the same time to be received and analyzed at the position of an ID transponder.
The ID transponder has 3D antennas for this purpose. Such 3D antennas are standard components, which are often used in ID transponders. These 3D antenna modules have multiple antenna coils in different spatial orientations and can detect signal strengths of electromagnetic signals in three different spatial directions.
In said document, different antennas are activated on the motor vehicle, which inevitably have different orientations to the mobile ID transponder.
It is then checked whether the respective signal strengths received in different spatial directions are consistent with the antenna positions on the motor vehicle. In addition, the targeted transmission strengths of the antennas can be varied so as to cause angular shifts of the resulting field vector.
If an extension takes place via one or more relay stations, the maintenance of this information for the orientation of the electromagnetic field is hardly possible. Even if the information were detected in the first, near-vehicle relay station, the position of the relay stations to one another is not to be predetermined, and also, not the position of the ID transponder to the ID transponder-side relay station.
The key will be determined according to the inconsistency between the known positions of the antennas on the motor vehicle and the received signals, and can deduce a relay attack.
Depending on the position of an ID transponder to the motor vehicle, the signal strength can be changed significantly. This is the case e.g., where if an ID transponder is in an unfavorable position relative to one of the used transmission antennas, it comes to overrides or low signal strengths, or even if the ID transponder is placed in a point, the field vectors of the simultaneously operated antennas are canceled.
The invention relates to a further development of the system and the method of said WO 2015/084852 A1.
Accordingly, reference is made to this document with respect to the basic principle of the function of the evaluation of the field vectors and orientation-dependent signal strengths.
The object of the invention is to design a method for detecting relay attacks reliably and robustly.